Technological development and increased demand for mobile equipment have led to a sharp increase in the demand for secondary batteries as energy sources. Among these secondary batteries, lithium secondary batteries having high energy density and driving voltage, long lifespan and low self-discharge are commercially available and widely used.
In addition, in recent years, increased interest in environmental issues has brought about a great deal of research associated with electric vehicles (EVs) and hybrid electric vehicles (HEVs) as substitutes for vehicles, such as gasoline vehicles and diesel vehicles, using fossil fuels which are major causes of air pollution.
Nickel metal hydride (Ni—MH) secondary batteries or lithium secondary batteries having high energy density, high discharge voltage and power stability are generally used as power sources of electric vehicles (EVs), hybrid electric vehicles (HEVs) and the like.
Lithium secondary batteries used for electric vehicles should have high energy density, exert high power within a short time and last for 10 years or longer under harsh conditions, thus requiring considerably superior stability and long lifespan, as compared to conventional small lithium secondary batteries.
In addition, secondary batteries used for electric vehicles (EVs), hybrid electric vehicles (HEVs) and the like require rate characteristics and power characteristics according to driving conditions of vehicles.
At present, as cathode active materials for lithium ion secondary batteries, lithium-containing cobalt oxide having a layered structure, such as LiCoO2, lithium-containing nickel oxide having a layered structure, such as LiNiO2, and lithium-containing manganese oxide having a spinel crystal structure, such as LiMn2O4 are used. A graphite material is generally used as an anode active material.
LiCoO2 is currently used owing to superior physical properties such as cycle properties, but has disadvantages of low stability, high-cost due to use of cobalt, which suffers from natural resource limitations, and restriction of mass-use as a power source for electric automobiles. LiNiO2 is unsuitable for practical application to mass-production at a reasonable cost due to many factors associated with preparation methods thereof.
On the other hand, lithium manganese oxides such as LiMnO2 and LiMn2O4 have an advantage of use of manganese which is abundant as a raw material and is eco-friendly, thus attracting considerable attention as a cathode active material capable of replacing LiCoO2. However, lithium manganese oxide also has a disadvantage of poor cycle properties.
LiMnO2 disadvantageously has a low initial capacity and requires scores of charge/discharge cycles so as to obtain a predetermined capacity. In addition, LiMn2O4 suffers rapid capacity deterioration in cycle life and, in particular, disadvantageously causes sharp deterioration in cycle properties at a high temperature of 50° C. or higher due to decomposition of electrolyte and elution of manganese.
In this regard, Japanese Patent Application Publication No. 2003-086180 discloses a method for improving charge/discharge cycle properties by adjusting a mean oxidation number of manganese ions to 3.03 to 3.08 through substitution of a part of oxygen of LiMnO2 by a halogen element.
In addition, Japanese Patent Application Publication No. 1999-307098 discloses a method for improving high-temperature cycle properties by substituting a part of oxygen of LiMn2O4 by a fluorine element.
In addition, Japanese Patent No. 3141858 discloses a method for improving power, energy density and cycle properties by coating the surface of active material particles such as LiMnO2 and LiMn2O4 with a metal halogenized material and substituting oxygen in the particles by a halogen element to prepare a solid solution.
However, lithium manganese oxides such as LiMnO2 and LiMn2O4 cannot secure a desired level of safety and have limitations as to improvement in energy density due to their crystalline structure in spite of these conventional methods.
Meanwhile, the lithium-containing manganese oxide includes Li2MnO3, in addition to LiMnO2 and LiMn2O4. Li2MnO3 is unsuitable for use in a cathode active material for secondary batteries due to electrochemical inertness, in spite of considerably superior structural stability.
Accordingly, some conventional methods suggest solid solution treatment or mixing of Li2MnO3 with LiMO2 (M=Co, Ni, Ni0.5Mn0.5, Mn). These cathode active materials have a broad domain in a high voltage region of 4.3V to 4.6V. This broad domain is known as a range in which lithium (Li) and oxygen (O) are deintercalated (left) from a crystal structure of Li2MnO3 and lithium is inserted into an anode.
The deintercalation of lithium and oxygen in the high voltage range of 4.3V to 4.6V imparts electrochemical activity to active materials and the broad region increases capacity, but decomposition of electrolyte and generation of gas may readily occur at high voltage due to oxygen gas generated in the battery, crystal structures are physically and chemically deformed during repeated charge/discharge, rate properties are deteriorated and, as a result, battery performance is disadvantageously deteriorated.
In addition, the cathode active material does not contribute to capacity due to lowered terminal region of discharge voltage when used for cellular phones, or it cannot practically realize high power, since it exhibits an unusable stage of charge (SOC) due to low power when used for vehicles.
Accordingly, there is an increasing need for methods capable of ultimately solving these problems.